Thanks for the advice.
Even though this Duet 2 Wifi card is pretty old (having come to me out of a different printer that was headed to the scrapheap), it seems to be new enough to have a fan fuse.
Although it looked ok, putting my multimeter across the fuse blades showed it is kaput.
I’ll swap it out and see how we go.
Thanks again
Yes - I can confirm that by “set correctly” I mean that there is no jumper.
I’ll have a look and see what I can figure out, but to be honest, I’m a bit dirty that I only just got this printer built, working and dialled in, and now I have to pull it apart again just as I got to the fun bit. 
That said, I’m happy with the Smart Effector, except for this failure. I think it has become an essential component in any decent Delta build.